As this type of apparatus, there has been suggested an apparatus using the EGR apparatus to remove toxic substances in exhaust gas (e.g. refer to Non-Patent document 1). According to the EGR apparatus of the internal combustion engine disclosed in the non-patent document 1, at the time of low-temperature warm-up in which the temperature of a catalyst is lower than a temperature at which aldehyde can be removed, the exhaust gas is recirculated or returned from a downstream portion of the catalyst, and if the temperature of the catalyst is higher than the temperature at which aldehyde can be removed, the exhaust gas is recirculated from an upstream portion from the catalyst. Thus, it is considered that the emission of aldehyde in the low-temperature warm-up can be significantly reduced.
Incidentally, there has been also suggested an ignition timing control method for a natural gas vehicle, which suppresses exhaust temperature reduction by delaying ignition timing from optimal ignition timing by a predetermined period of time if the temperature of an exhaust gas purifying catalyst detected during idling or within a fixed time after the idling is lower than a catalyst active temperature (refer to Patent document 1). According to this apparatus, it is possible to effectively remove methane in the exhaust gas.